Advanced energy storage systems such as lithium-ion batteries are important approaches to mitigate energy shortage and global climate warming issues that the world is currently facing. High power and high energy density are essential to batteries for applications in electric vehicles, stationary energy storage systems for solar and wind energy, as well as smart grids. Because conventional lithium-ion batteries are inadequate to meet these needs, there is an urgent need for new electrochemical cell chemistries to achieve increased electrical energy storage capacity and rate capability for future energy storage needs. Yet, few viable new electrode materials have emerged. A highly attractive approach is to utilize a redox couple involving multiple oxidation states of the electrode material, as has been demonstrated in metal fluoride conversion electrodes (e.g., FeF3). The ionic nature of the metal-fluoride bond results in a high reaction potential (EMF) when coupled with lithium, which in combination with the ability to access all energetically favorable transition metal oxidation states (e.g. Fe3+ to) Fe0, leads to a large theoretical capacity for lithium ion batteries. However, use of metal fluoride materials has been hindered in the past by irreversibility in the conversion reaction and poor conductivity. These problems are linked with poor conductivity, and only a very small current can be used to obtain a large reversible capacity.
Studies have shown that small separation distances between the LiF/metal products formed during cycling of metal fluorides (i.e., a nanocomposite) can promote a reversible reaction. The formation of nanocomposites provided the first breakthrough in achieving a cyclable lithium ion battery with metal fluoride electrodes. In general, approaches that have been used to improve reversible capacity of conversion electrode include: (1) forming a nanocomposite with carbon to improve electrical contact; (2) coating with oxides to control the interface between the electrode and electrolyte (a “mixed conducting matrix”, or MOM); and (3) formation of an architecturally controlled, crystalline mesoporous electrode to lower Li+ diffusion distances. Similar approaches have been used to improve the electrochemical performance of the intensively studied cathode LiFePO4, which was not considered technologically relevant as a pure micron-sized powder. In that case, a combination of decreased particle size and coating with electronically conducting agents has been used overcome electronic and ionic transport limitations. Furthermore, very high rate performance has been reported in LiFePO4 through coating with a fast ion conducting Li4P2O7-like phase. The Li4P2O7-like phase likely aids in the rapid delivery of lithium to surface sites on the LiFePO4 particles. However, the practical capacity of LiFePO4 is ˜150 mAh/g, as compared to reported capacities for FeF3 in the range of ˜270-600 mAh/g. The major limitation for metal fluoride conversion electrodes has thus far been an inability to couple high capacity with a high rate capability and cycle life (reversibility). Oxide based modification to form FeOF has been shown to improve the reversible capacity of FeF3 over many battery cycles, but does not sufficiently improve the conductivity to obtain a high rate capability.
Therefore, it remains highly desirable to develop an electrode with improved conductivity, improved capacity retention with high rate capability, and improved cycle life.